<!DOCTYPE html>
<html lang="en">
<head>
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  <title>Document</title>
</head>
<body>
  <div class="box">
    5XX
  </div>
</body>
</html>

<style>
* {
  margin: 0;
  padding: 0;
}
html, body {
  width: 100%;
  height: 100%;
}
.box{
  width: 100%;
  height: 100%;
}
.img{
  width: 100%;
  height: 100%;
}
</style>